Allow to use unix socket for non-default servers

It allows to mitigate the error 'Address already in use' while binding
admin (console) port. Check [1] for more information (see comment
itself, the issue is about the another problem).

[1]: #115 (comment)

lib/preprocessor.py

@@ -211,6 +211,7 @@ def server_create(self, ctype, sname, opts):
         if 'rpl_master' in opts:
             temp.rpl_master = self.servers[opts['rpl_master']]
         temp.vardir = self.suite_ini['vardir']
+        temp.use_unix_sockets = self.suite_ini.get('use_unix_sockets', False)
         temp.inspector_port = int(self.suite_ini.get(
             'inspector_port', temp.DEFAULT_INSPECTOR
         ))

lib/tarantool_server.py

@@ -514,7 +514,9 @@ def install(self, silent=True):
         self.copy_files()
 
         if self.use_unix_sockets:
-            self._admin = os.path.join(self.vardir, "socket-admin")
+            path = os.path.join(self.vardir, self.name + ".socket-admin")
+            warn_unix_socket(path)
+            self._admin = path
         else:
             self._admin = find_port()